This invention relates to an access manhole that is mounted in a negative pressure storage or process tank or the like and, in particular, to a method of replacing a manhole cover including a bolted-on blind flange with a manhole cover including a hinged door assembly.
Many negative pressure storage or process tanks and, in particular, older tanks, are equipped with manholes that include covers that open outwardly with regard to the tank. Typically, the cover is bolted to a flange that encircles the outside of a cylindrical manway. The term manway, as herein used, refers to a tubular member that is welded or otherwise joined to the tank to provide access into the tank.
Replacing a manhole with a bolted-on blind flange within a negative pressure storage tank with a more preferred hinged door assembly has heretofore presented certain structural difficulties where the existing manway is removed from the tank and replaced with an entirely new manhole unit.
Complete removal of the existing manhole can produce structural damage to the tank shell in and about the manway opening that oftentimes requires extensive repair to the tank. Furthermore, retrofitting a new manway to the repaired opening is sometimes difficult and can result in damage to existing tank coatings, linings or insulation.
Turning initially to FIGS. 1 and 1A, there is shown a typical negative pressure manhole MH which is constructed in accordance with the American Petroleum Institute (A.P.I.) Standard 650 welded steel tanks for oil storage.
The manhole MH includes a manway MH that surrounds an opening O formed in the tank shell TS that communicates with the interior I of the tank. The manway MW contains a cylindrical body section CB having an inner flange IF which is welded to the tank shell TS and an outer bolting flange BF. A cover C is secured to the outer flange by a number of bolts B. A gasket G is generally mounted between the outer flange and the cover C to prevent leakage from the tank to the surrounding environment. Although the manway MW is shown oriented vertically, the actual orientation could vary anywhere from vertical upward opening to horizontal to vertical downward opening.
The manhole has certain disadvantages with regard to opening and closing, and it is oftentimes desirable to replace the manholes having bolted-on blind flanges with ones that include a hinged door assembly. The conversion, however, can cause damage to a steel tank or weaken the tank, particularly in the case where the existing manway is cut away from the tank shell and replaced with a completely new unit. In the case of a tank constructed of refractory bricks or the like, damage can also be extensive and the amount of repairs required to place the tank back in service is rather costly.
As will be described in detail below, the present invention relates to a method of converting an existing manhole having a bolted-on blind flange with one that has a hinged door assembly so that it causes little or no damage to either a steel or refractory tank.
The inventive method of converting a tank is also more cost effective that those previously employed in the industry.
It is therefore an object of the present invention to provide an improved method of converting existing manholes in negative pressure storage tanks or the like so that the cover of the manhole is opened and closed using a hinged door assembly rather than unbolting and bolting a blind flange.
A further object of the present invention is to replace an existing manhole in a negative pressure storage tank with a minimum of damage or no damage being done to the tank structure.
A still further object of the present invention is to reduce the amount of time required to replace a manhole with a bolted-on blind flange in a negative pressure storage tank with one having a hinged door assembly.
These and other objects of the present invention are attained by converting an existing manhole in a tank which has a blind flange cover mounted upon a bolting flange with a cover unit including a hinged door assembly. The conversion is accomplished by removing the blind flange from the bolting flange of the existing manway. A hinged door assembly is joined to a mounting flange and the mounting flange is attached to the existing bolting flange. An outwardly opening cover unit is rotatably mounted upon the hinge assembly so that it closes against the rim of the manway.